Abstract - Properties of fructose 1,6-diphosphate independent -lactate dehybrogenases from steptococcus thermophilusThe properties of S. thermophilus LDH have been studied using soluble crude extracts of 10 strains and also partially purified extracts of 2 of them. The stability of LDH when refrigerated and frozen as well as its thermoresistance varied with the strains. Phosphate activated LDH but FDP did not; on the contrary FDP inhibited the LDH activity at concentration > 1 mM. Slight inhibition of LDH occurred with Mn+ +. The ADP, ATP, 6-P-gluconate, oxamate and NAD were the strongest inhibitors of LDH activity measured with pyruvate as the substrate. Some of the inhibitions varied with the pH. The activity with L(+)-lactate as the substrate was 30 times lower than the reverse activity (reduction of pyruvate) and lactate concentrations above 200 mM were needed. An excess of NADH (0.2 mM with strain 302; 0.33 mM with strain 385) inhibited the LDH activity. Likewise, pyruvate (> 10 mM) was inhibitory but its effect was different according to the presence or absence of FDP (0.5 mM). The Km and Vmax constants were measured. The activity of LDH remained unchanged between pH 4.7 and 7.5. However this activity rapidly decreased at pH < 4.7, becoming almost nil at pH 4.6. The S. thermophilus LDH electrophoretic mobilities were compared to those of the well known isoenzymes of animal origin. Two types of LDH with different mobilities were found: the A type, soluble in 55 p. 100 ammonium sulphate solution, and the B type (the most active) insoluble at this concentration.These results confirm the physiological, nutritional and metabolic particularities of S. thermophilus species and also the variations found among the strains within the species